Know where you are in relation to other characters. Don't suddenly appear in somepony's bedroom, and don't try to respond to characters in two different places at the same time.

Never dictate another's actions. For instance, instead of saying you slap somepony, say you try to slap somepony, or you swing at somepony. Leave it up to them to decide if they manage to block or dodge your blow. You can always attempt to encourage a certain response (such as by saying you suddenly swing at them, or you clumsily swing at them), but they have final say over what happens to their character.

The RP takes place in Ponyville. You can live or visit wherever you want, but if you're not in Ponyville you will be isolated.

There are no religions. Celestia and Luna are princesses, not goddesses.

Ponyville has yet to be extensively touched by technology. Pinning down the exact tech level is difficult (they have steam engines and arcade machines), but the less you assume the better.

No clones.

Standard RP is pretty carefree, assuming a fairly utopian Equestria. If you go grimdark in standard RP, expect serious consequences (such as a week of real time spent in the hospital).

Epic RP is darker and overpowered. Here you can fight villains and monsters with swords and unicorn magic. Treat it as an alternate reality though: your adventures do not exist when your character chats with standard RPers (see below).

Be modest. You don't have to stand on a stage or talk in third person to be a Trixie. Standard RP ponies don't care if you can throw fireballs, are in the royal guard, or went to flight camp with Rainbow Dash. Mentioning them is generally just a way to try to get attention. Nothing says your character can't be or do anything impressive, but we can spot a Mary Sue a mile away.

Things to avoid mentioning in standard RP:

Being a professional adventurer like Daring Do

Being a revolutionary inventor

Being royalty

Being a ponified version of another character

Knowing more than a few talent-aligned magic spells

It's best to be a pony. There aren't an abundance of zebras, dragons, griffons, or humans living in Ponyville. Alicorns are against the rules.

Try to come up with a hope or goal for your character. It's all too easy to end up existing only for your shipping partner. It helps to have something outside of your character's relationships to tie them to the RP.

Try to come up with a personality flaw for your character. Almost every episode revolves around dealing with somepony's flaw.

Come up with a special talent. This is probably closely tied with your character's hope/goal. You should probably do this even if your character is a blank flank and hasn't discovered their talent yet. Try to avoid cliches like "my special talent is fire."

Trollplay is something being so far out of the ordinary that it cannot fit in canon, such as breaking the fourth wall, using modern technology, or acting obviously out of character for no reason. The purpose of trollplay is to be silly.

Characters can interact with trollplay, and immediately forget about it when it stops, allowing some fun while avoiding lingering effects of nonsense.

Characters do not need to recognize or dignify trollplay.

Some characters (such as CreamCake) exist for trollplay, while others (such as Pinkie) only occasionally trollplay and are otherwise normal.